Winds are generally caused by the movement of air molecules. The movement of air molecules decides the type of wind; whether it should be a calm breeze or a violent storm. If the air moves faster from one place to another it creates a violent storm whereas if it moves slowly, it will make a calm breeze. But what creates this movement of air molecules? Let’s find out.
The main cause for the movement of air molecules is due to the difference in the atmospheric pressure. The atmospheric pressure is defined as the pressure exerted by the atmosphere, on the surface of the earth. Our planet Earth is surrounded by a layer of gas called the atmosphere. The atmosphere is comprised of various compositions of gases. Due to the gravitational force exerted by the earth, the air molecules surrounding the earth are pulled inwards. So the air present in the atmosphere exerts a tremendous pressure on the earth's surface. The atmospheric pressure that is exerted on the earth is equal to 14.7 pounds per square inch (psi).
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Atmospheric Pressure over Earth |
This pressure is also exerted on every object, present on the earth's surface. Such pressure makes the object in the earth to get crushed. But in the meantime air pressure present inside everybody balances it from the outside pressure, which makes the thing not be crushed by the outside air. The best example for this, is the pressure range in the human body. As the pressure exerted by the atmosphere is too much we could get crushed by it. But as the air is also present in our body, the pressure range between inside and outside of our body becomes equal and we are protected from the outside air pressure. Similarly, as there is no air present in the outer space, the air pressure present inside our body will be greater than the outside pressure, and our body will get bulge. So to get the equivalent amount of pressure in space, astronauts wear space suits, which will give the outside pressure to the astronauts. So the earth and every object on the earth experience an equal amount of pressure; both on inside and outside of them. When there is a change in pressure on the earth's surface; the airflow begins to manage the pressure.
Let’s assume a bright sunny day with warm weather. Due to the radiation emitted by the sun, the land gets warmer and the air present near the ground gets heated up. Due to this, the air molecules present on the ground become less dense and get expand. Thus it begins to move up, and the sky looks cloudy. Due to the absence of air at the ground level, a low pressure is created on the earth's surface, which means the pressure exerted by the atmosphere is low. Similarly consider an instance where there is a cold temperature. Due to this, the air molecules present in the sky, join together and becomes highly dense, and begin to sink down, by which the sky looks clear. Due to this action, high atmospheric pressure is created on the earth's surface. In this instance, air molecules move from a high-pressure area to low pressure area. Thus the movement of the air molecules between these two areas creates a wind. The reason for getting a heavy breeze on the coastal side is also due to this huge pressure difference. Let’s see how this happens with an example,
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Pressure Difference |
When we keep a cup of sand and a cup of water in bright sunlight, the sand cup gets hotter than the water cup. Likewise, on earth, the land gets hotter compared to the sea. So the air present in the land gets warmed up and moves upwards, thus creating low pressure on the land area. But as the air present at the seaside is cold, it gets sinks and maintains its place near the sea surface, thus creating a high-pressure area. So due to the pressure difference between these two areas, the air present upon the sea moves towards the land creating a wind. As the pressure difference on the coastal side is more, it creates a strong breeze. When even more pressure difference is created, it causes major destruction. One of the major destruction is the hurricane and it is mainly caused by the pressure difference and the Coriolis Effect. Let’s also take a shorter look at the Coriolis Effect which plays a major role in global air circulation.
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Pressure Difference |
The Coriolis Effect happens due to the spinning of the earth. As the earth spins, the equator experiences a much higher speed compared to the poles. So the wind present on the equator moves much faster, while the wind present on the poles moves much slower.
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| Coriolis Force in hurricane creation |
When low pressure is created between the poles and the equator, air from the surrounding high-pressure regions is moved into the low-pressure region. If the air from the equator moves to the low-pressure area, it is always propelled ahead; because the areas around the equator are moving much slower than the equator. Similarly, when the air from the pole moves to the low-pressure area, it is propelled backward (lagged); because the area around the pole is moving much faster than the pole regions. This both leading and lagging air create a rotating motion for the movement of the air; which creates a hurricane. The direction of a hurricane will be in an anticlockwise direction in the northern hemisphere whereas in the southern hemisphere the direction of the hurricane will be the opposite direction as the deflection of air will be opposite in the southern part.
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Credit (Hurricane in Northern Hemisphere): By NASA-GSFC, data from NOAA GOES - http://rsd.gsfc.nasa.gov/goes/pub/goes/luis.9509091445G8I01.gif, Public Domain, https://commons.wikimedia.org/w/index.php?curid=173183 |
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